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A loud, dumb face you make when you see something so stupid it makes your brain want to explode.
I saw my dog wearing a hat and I did a hachi machi so loud my neighbor called the cops.
My mom said I could eat ice cream for breakfast and I did a hachi machi so hard I spilled my cereal.
When my friend told me he got a fish for a pet, I did a hachi machi so strong my face turned red.